This is a minor issue, but I was experimenting with how the gutter behaves as you resize the window and I think there’s a minor CSS mismatch.
At the 870px width @media, the topic-body jumps from width: 690px to width: calc(100%-47px), which, combined with some other changes to the layout, makes the topics suddenly get wider. When you further narrow the window to ~795px, the calculation catches up with the old value, and then shrinks as expected.
In topics with long posts, this can cause quite jarring popping of posts up and down the page.
This can be “fixed” by altering the @media to trigger at 795px instead of 870px. That doesn’t feel very elegant though - I’m sure the 870 value was chosen for a reason!